This invention relates to apparatus in electrostatographic copiers or printers for fusing toner images (carried on a suitable receiver or copy sheet) at a desired fusing temperature. More particularly, the present invention relates to a heated fusing apparatus that effectively prevents "droop" or a dropping from such a desired fusing temperature, and that substantially reduces heating of the receiver or copy sheet, thereby preventing the occurrence of heat-related copy sheet defects such as curling, blistering, and image offset.
In electrostatographic copiers and printers, it is well known to use a heated fusing apparatus, for example a heated fuser and pressure roller type apparatus, at a desired fusing temperature, to fuse toner images to a suitable receiver or copy sheet of paper. Normally, such apparatus is operated through a run period when toner images on a receiver or copy sheet are fused, and through a standby period when the apparatus is awaiting a run period. In such apparatus, the pressure roller, which forms a heat transfer or fusing nip with the heated fuser roller, typically includes a rigid thermally conductive shell that can store heat received from the heated fuser roller.
On the other hand, the fuser roller which may include a metallic core that is coated with a thick layer of an elastomeric material, may be heated externally, or internally by a heat source. When heated internally, the heat source is located within the metallic core, and heat therefore must flow from within such core, outwards through its elastomeric coating to its surface, and then across the heat-transfer or fusing nip to the pressure roller.
During the run periods, some of such heat, however, will be absorbed by, and first heat, any toner image-carrying receiver or copy sheet within the nip, before it then reaches the pressure roller. In order to sustain such heat flow, the heat source of the fuser roller must be turned on and off such that the core of the fuser roller remains relatively hotter than its surface, as well as, than the pressure roller. However, if the surface of the pressure roller became relatively hotter than the toner image-carrying receiver or copy sheet being run, heat will additionally flow back, from the pressure roller, into such a receiver or copy sheet.
When using such apparatus for fusing toner images at a desired fusing temperature, such a temperature typically is measured on the surface of the fuser roller, and is controlled by turning the heat source on and off in response to changes in such measured temperature. Normally, in response to such temperature changes, a reduced duty cycle may be initiated for the heat source, or the heat source may be turned off completely, during standby periods. As a result, the duty cycle must be increased or the heat source turned back on, in response to the measured temperature dropping below a particular control setpoint, for example, the fusing setpoint.
"Droop" or a continued dropping from the desired fusing temperature setpoint of the apparatus occurs in part because of such on/off control of the heat source. This is because reducing and increasing the duty cycle, or turning the heat source on and off, as such, does not instantly begin to raise or lower the temperature of the surface of the fuser roller. Instead, when the heat source is turned on in response to the temperature of the fuser roller surface dropping below the desired fusing setpoint, for example, the immediate result is an undesirable continued dropping or "drooping" from such a setpoint. Such continued dropping or "droop" is due in large part to the thermal capacitances and inertia of the core and the elastomeric coating of the fuser roller, and is especially undesirable because it can introduce variations in fusing quality, and if severe, can result in poor and unacceptable images.
Unfortunately too, raising the temperature control setpoint at which to turn the heat source on and off in an attempt to avoid such a "droop" or "drooping" problem may undesirably result in the pressure roller becoming overheated during the standby periods. Such an overheated pressure roller will substantially increase the amount of heat that will be transferred back by such pressure roller to the receivers or copy sheets being run, for example, at the start of an ensuing run period. Such backtransfer of heat will equally overheat the receivers or copy sheets, and consequently risk the occurrence of heat-related defects such as curling, blistering, and image offset.